In communication systems a modem is used to convert (modulate) digital signals generated by a computer into analog signals suitable for transmission over telephone lines. Another modem, located at the receiving end of the transmission, converts (demodulates) the analog signals back into digital form. Timing recovery refers to, and includes, the generation and/or reconstruction of a clock signal, at a receiver, to sample the incoming signal. The clock signal generated at the receiver end must agree in frequency with the transmitter clock at the sending end, and may also require proper phasing. Timing recovery includes techniques that make use of the received waveform to help recover timing.
To recover data from a Quadrature Amplitude Modulated (QAM) signal (for example), a receiver is required to convert the analog signal received into a digital signal and to distinguish between the individual symbols that comprise the signal bitstream. Symbol timing recovery is used to recover a clock signal at the symbol rate, or a multiple of the symbol rate, from the modulated waveform that is received. This clock signal may be used by the receiver to convert the continuous-time received signal into a discrete-time sequence of data symbols. Traditional recovery circuits are typically functionally limited to a small operating range. When the signal loss and distortion vary over a large dynamic range, the behaviour of the circuit changes dramatically due to the large variability in the circuits s-curve maximum amplitude for different channels.
An s-curve is a curve showing the output of a phase detector in a timing recovery circuit as the phase difference between a received signal phase and an ideal phase between 0 and 2.pi.. The amplitude of the s-curve can be viewed as a measure of the quality of the timing recovery. In particular, robust and accurate timing recovery in a modem receiver generates a large maximum amplitude of the s-curve. If the s-curve maximum amplitude is reduced (due to loss, distortion, etc.) it results in long acquisition times, and large phase jitter after acquisition. This problem is exacerbated by the use of various QAM constellations, slope equalizer settings, and by the presence of radio frequency interference (RFI) such as from commercial AM broadcasts.
These characteristics of recovery circuits make it difficult to determine when the recovery circuit has converged (i.e. when the circuit acquires lock). Circuit lock refers to obtaining a frequency match between a receiver oscillator to a transmitter clock. The phase (sampling phase) of the receiver clock is not the same as the transmitter clock, but it is the optimum phase that allows the recovery of the data. The optimum phase varies depending on the channel response.